S4 II. ORDERS AND GENERA. 



other orders rather hy small and few technical distinc- 

 tions, than by any very decided characters. They are in 

 fact groups bearing an intermediate value between genera, 

 on the one hand, and well-marked assemblages such as 

 Filices, Gramina, or Palmi, on the other hand. 



Looking to Britain only, the orders named in the sixth 

 line may be described as three pairs of genera, which are 

 tied together passably well by mutual resemblance ; but 

 which cannot be so closely tied to any other orders, and 

 are therefore treated as orders of themselves. When we 

 look beyond Britain, they gain little accession of generic 

 numbers ; the Typhacece none at all. They are not 

 orders constituted by a central type, to which a number 

 of other genera stand in more or less close affinity, but 

 always more similar to the type than to anything else. 

 Typha and Sparganium are co-equals ; and it would be 

 mere caprice to affirm that either one is the tj-pical genus 

 of its order, from which the other is an aberrant genus. 



In the seventh line, three very small groups are named. 

 The two first of these are merely single genera which now 

 stand as orders by themselves. Systematists have en- 

 deavoured to combine them into an ordinal group with 

 other genera; as, for example, with Myriophyllum and 

 Hippuris, and also some foreign genera, forming together 

 a very heterogeneous assemblage in proportion to their 

 numbers. The genus Elatine, though the only represen- 

 tative of its order in Britain, is not so in the general sys- 

 tem ; some other inconspicuous foreign genera being 

 united with it, to constitute the order Elatinacem. We 

 must thus look upon only the two first of the three 

 groups as being really single genera, which are called 

 orders of themselves because botanists are at a loss what 

 else to do with them in systematic groupings. ' 



The twenty-one orders thus selected for illustration. 



